Copper as major industrial metal is recovered from naturally occurring ore deposits. These deposits may contain copper in the form of sulfides such as chalcopyrite, bornite, chalcocite and covellite, or oxides such as cuprite or tenovite, the hydroxy carbonates such as malachite or azurite, or as the silicates such as chrysocolla. The grade of such deposits has decreased as the richer deposits have been mined over the years, and it is not unusual for deposits containing as little as 0.4 percent copper to be mined today. Accordingly, the mining and transporting of the massive amounts of rock necessary for the recovery of copper requires comparatively large amounts of energy for the amount of copper recovered.
The established method of recovering copper from such low grade deposits has involved mining the deposits and then grinding the ore to a fine size to permit floating the liberated copper minerals. In cases where the mineralization is primarily oxides or silicates that are not amenable to flotation, the ore is often leached with acids to recover the copper. However, such acid leaching is not effective in recovering highly insoluble copper sulfide which may also be present and is in general susceptible to only a very poor recovery of the copper. Indeed, recoveries of more than 50 percent of the copper present in the copper sulfide ore are unusual and, when attained, frequently involve the fine grinding of the ore with a large expenditure of energy.
In the processing of copper sulfide ores it is frequently necessary to grind more than 90 percent of the rock to a screen mesh size of minus 60 mesh in order to liberate the copper sulfide particles for flotation. In doing this a great amount of energy is expended driving huge ball mills. Additionally, the grinding of the rock to such a small size results in the production of a large quantity of copper sulfides which are very fine, often finer than ten microns, and thus not recoverable by flotation. Indeed, recoveries of copper from low grade ores using flotation techniques are seldom over 90 percent.
When the copper sulfides are isolated by flotation they must still be smelted or otherwise converted to copper. The processes used frequently involve large amounts of energy to initiate chemical reactions.
The entire sequence of events leading to the recovery of copper from its ores and its conversion into metal is characterized by the expenditure of vast amounts of energy and complexity of processing. It would obviously be very desirable to minimize the effort put into breaking up the gangues with which the ore exists and put that energy exclusively into the copper compounds which one wishes to recover or convert.
Microwaves are well known for their use in radar and in communication transmission. They have been extensively used as a source of energy for cooking food. Although microwaves have been studied for many years and put to practical uses, the effects which they may have on many materials are not known. The effects of microwaves on many ores and minerals are not known, nor can they be readily predicted. The effects of microwaves on metal values contained with ores does not appear to be related in any predictable way to the chemical or physical properties of such metal values. For example, it has been found that copper in its oxide, sulfide or silicate forms is very susceptible to heating by microwaves of 915 megahertz or 2450 megahertz, whereas zinc oxide or sulfide does not respond, or responds only slightly. Likewise, it has been found that the sulfides of molybdenum and rhenium absorb microwaves. It has also been found that nickel, cobalt and manganese oxides absorb microwaves, but the oxides of iron and chromium, which are transition metals, do not absorb microwaves.
By the use of microwaves as herein disclosed, one can selectively heat the copper (whether oxidic or sulfidic) without the necessity of heating the whole rock mass, because the gangue is substantially transparent to microwave radiation while the copper minerals are very effective in absorbing the microwaves.
U.S. Pat. No. 2,733,983 to Daubenspeck teaches the use of ferric chloride at high temperatures of 600.degree. C. to 700.degree. C. to chlorinate nickel and cobalt oxides. U.S. Pat. No. 4,144,056 to Kruesi discloses heating a metal oxide or silicate in the absence of air with ferric chloride and a volatility depressant salt selected from the group consisting of alkali metal chlorides and ammonium chlorides for a time of about 30 minutes to about 1 hour at temperatures of from about 200.degree. C. to about 600.degree. C. Conventional heat sources are used in both processes where heat is required.
U.S. Pat. Nos. 4,123,230 and 4,148,614, both to Kirkbride, disclose the desulfurization of coal by subjecting the coal or slurry of coal particles in a hydrogen atmosphere to microwave energy to form hydrogen sulfide which is removed from the coal with solvents. U.S. Pat. No. 4,152,120 to Zavitsanos, et al, removes pyritic and organic sulfur from coal by mixing alkali metals or alkaline earth compounds with the coal and using microwave energy to selectively heat these compounds and the sulfur to convert organic and pyritic sulfur to soluble alkali and alkaline earth compounds which are removed from the coal. The subject matter of this patent is also disclosed in an article entitled "Coal Desulfurization Using Microwave Energy," Zavitsanus et al, published in U.S. Department of Commerce PB 285-880, June, 1978. This patent and the article teach the use of microwave energy to selectively heat pyritic and organic sulfur contained in the organic host material coal in the presence of other elements or compounds to convert the sulfur into soluble compounds which can be readily removed from the coal. They do not teach the use of microwave energy to selectively heat metal compounds in their inorganic mineral-like host materials, alone or in the presence of other elements or compounds, to form soluble compounds of the metals which are readily recoverable from the host material. Particularly, they do not teach the unexpected finding that the process will work on certain ores or minerals containing metal values and not on other ores and minerals to recover their metal values.
In treating copper ores it is only the copper compounds which are appreciably heated; the gangue of the ore does not appreciably absorb microwave radiation. None of the prior art recognizes the characteristic of the sulfides, oxides, hydroxy carbonates and silicates of copper to absorb microwaves or the fact that the accompanying gangue is low absorbant, transparent to and/or reflective of the microwave energy.